IMS Architecture and Session Control Functions
TISPAN/IMS Architecture
In Release 6 of the 3GPP specifications, the IMS network architecure has been defined to be access independent. This means that the access technology used to transport user SIP messages to the IMS network does not impact the functionality of the IMS network itself. Consequently, any access can be used, examples being DSL, Cable, WLAN, GPRS, etc. This is obviously a key step in the move towards converged network architectures.
The IP Multimedia Subsystem itself is made up of a number of component 'blocks' as shown below.
Figure 1- IMS Network Architecture - Functional Elements
To simplify matters, items above in green are associated with PSTN egress and ingress and items in burgundy are associated with IP-to-IP SIP calls. The key functions are as follows
P-CSCF: The Proxy Call Session Control Function. This is the first point of contact within the IMS for a User Element (UE). The P-CSCF may be located in the home or visited network. In the context of fixed networks, the home network may be a wholesale network and the visited network may be a retail network that uses the wholesale network. The P-CSCF ensures that SIP registration is passed to the correct home network and that SIP session messages are passed to the correct Serving CSCF (S-CSCF) once registration has occurred. Contact with the home network during registration is through the home network I-CSCF and initial SIP session set-up is through called party I-CSCF.
The P-CSCF is an important function as it is in the position to detect services, which should be hosted by the visitor's network. This is important not only in terms of content services but also in the provision of Emergency Call Handling and Lawful Intercept. It also can provide defence against SIP signalling attacks.
PDF: Policy Decision Function. (Also known as Service-based Policy Decision Function in TISPAN). This function takes a service level policy request from the application layer (for example P-CSCF) and translates it into IP QoS parameters. For example, a G.711 call would be translated into real-time priority with 80 kbps IP bandwidth requirement. The access network is then asked if it can provide this QoS. What happens next will depend on the type of access network used. In GPRS networks, the PDF will use the 'Go' interface to set the policing policy for that session in the GGSN. In TISPAN-based networks, the PDF contacts the Border Gateway Function (BGF) to enforce the policy. There is an important difference here: GGSNs are not SIP session aware and hence they can only police on PDP sessions. SIP expects sessions to be able to handle multiple media simultaneously and this is not the case here. It means that 3GPP has had to define an additional parameter, which forces the session to set up a new PDP context for each new media stream in the session. Border Gateway Functions, however, are controlled from the application layer to enable a new media pinhole to be opened for each media component. Policing can therefore occur on a per-media component basis. This will allow the operator to ensure that the users' media traffic is throttled or discarded if it exceeds the policy negotiated at the SIP layer.
I-BCF: Interconnect Border Control Function. This function was introduced by TISPAN to formalize interconnect between networks. Its roles include the provision of NAPT and firewall functions for signalling, policing of signalling, topology hiding and conversion between IPv4 and IPv6. Whilst 3GPP networks are IPv6 only, 3GPP2 and wireline evolutions are likely to be a mixture of IPv4 and IPv6. The standard is also likely to specify the detection of failure of interconnect points. Through the control of a Border Gateway Function, the IBCF also controls the media exchanged across the operator boundary, controlling media pinhole establishment, policing, and dynamic NAPT and firewall functions.
I-CSCF: Interrogating Call Session Control Function. This is the function within the home network that is able to determine the S-CSCF with which a user should register. This is achieved by querying the Home Subscriber Server (HSS), which checks that the user is allowed to register in the originating network and returns an S-CSCF name and capability if this is the case. The I-CSCF is then able to contact the S-CSCF with the register. The I-CSCF function can be removed from the signalling path once it has been used to establish which S-CSCF is in use. The exception to this is if the THIG (Topology Hiding Inter-network Gateway) function of the I-CSCF is being used. The I-CSCF is also used in call set up in the called party's network to resolve the called party's S-CSCF.
S-CSCF: Serving Call Session Control Function. The S-CSCF is the function that registers the user and provides service to them (even though these services may be on separate application platforms). It performs routing and translation, provides billing information to mediation systems, maintains session timers, and interrogates the HSS to retrieve authorization, service triggering information and user profile. In short, it is the brain of the IMS.
HSS: Home Subscriber Server. The HSS is the database of all subscriber and service data. Parameters include user identity, allocated S-CSCF name, roaming profile, authentication parameters and service information. The HSS also provides the traditional Home Location Register (HLR) and Authentication Centre (AUC) functions. This allows the user to access the packet and circuit domains of the network initially, via IMSI authentication.
PSTN Ingress/Egress Functions. The PSTN ingress and egress functions allow interworking with the PSTN. Media Gateways provide the physical conversion between TDM and IP for media. Signalling Gateways provide translation at the transport layer between SS7 on MTP (TDM networks) and SS7 on SCTP (IP networks). The Breakout Gateway Control Function (BGCF) decides whether PSTN breakout will occur in the current network, or whether the SIP-I (ISUP over SIP) must first be sent to another IP network before breaking out. If it is the former case, then the signalling is passed to a Media Gateway Controller to then allocate a port on a Media Gateway for breakout. If it is the latter, the signalling is passed across the IBCF to another BGCF in the breakout network.
In the descriptions above, only the main functions of interest of the IMS architecture have been covered.
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